A murine model of falciparum-malaria by in vivo selection of competent strains in non-myelodepleted mice engrafted with human erythrocytes

Antimalarial drug discovery - the path towards eradication

Malaria is a disease that still affects a significant proportion of the global human population. Whilst advances have been made in lowering the numbers of cases and deaths, it is clear that a strategy based solely on disease control year on year, without reducing transmission and ultimately eradicating the parasite, is unsustainable. This article highlights the current mainstay treatments alongside a selection of emerging new clinical molecules from the portfolio of Medicines for Malaria Venture (MMV) and our partners. In each case, the key highlights from each research phase are described to demonstrate how these new potential medicines were discovered. Given the increased focus of the community on eradicating the disease, the strategy for next generation combination medicines that will provide such potential is explained.

A new in vivo screening paradigm to accelerate antimalarial drug discovery

The emergence of resistance to available antimalarials requires the urgent development of new medicines. The recent disclosure of several thousand compounds active in vitro against the erythrocyte stage of Plasmodium falciparum has been a major breakthrough, though converting these hits into new medicines challenges current strategies. A new in vivo screening concept was evaluated as a strategy to increase the speed and efficiency of drug discovery projects in malaria. The new in vivo screening concept was developed based on human disease parameters, i.e. parasitemia in the peripheral blood of patients on hospital admission and parasite reduction ratio (PRR), which were allometrically down-scaled into P. berghei-infected mice. Mice with an initial parasitemia (P0) of 1.5% were treated orally for two consecutive days and parasitemia measured 24 h after the second dose. The assay was optimized for detection of compounds able to stop parasite replication (PRR = 1) or induce parasite clearance (PRR >1) with statistical power >99% using only two mice per experimental group. In the P. berghei in vivo screening assay, the PRR of a set of eleven antimalarials with different mechanisms of action correlated with human-equivalent data. Subsequently, 590 compounds from the Tres Cantos Antimalarial Set with activity in vitro against P. falciparum were tested at 50 mg/kg (orally) in an assay format that allowed the evaluation of hundreds of compounds per month. The rate of compounds with detectable efficacy was 11.2% and about one third of active compounds showed in vivo efficacy comparable with the most potent antimalarials used clinically. High-throughput, high-content in vivo screening could rapidly select new compounds, dramatically speeding up the discovery of new antimalarial medicines. A global multilateral collaborative project aimed at screening the significant chemical diversity within the antimalarial in vitro hits described in the literature is a feasible task.

Synthesis and antiprotozoal activity of dicationic m-terphenyl and 1,3-dipyridylbenzene derivatives

4,4″-Diamidino-m-terphenyl (1) and 36 analogues were prepared and assayed in vitro against T rypanosoma brucei rhodesiense , Trypanosoma cruzi , Plasmodium falciparum , and Leishmania amazonensis . Twenty-three compounds were highly active against T. b. rhodesiense or P. falciparum. Most noteworthy were amidines 1, 10, and 11 with IC50 of 4 nM against T. b. rhodesiense, and dimethyltetrahydropyrimidinyl analogues 4 and 9 with IC50 values of ≤ 3 nM against P. falciparum. Bis-pyridylimidamide derivative 31 was 25 times more potent than benznidazole against T. cruzi and slightly more potent than amphotericin B against L. amazonensis. Terphenyldiamidine 1 and dipyridylbenzene analogues 23 and 25 each cured 4/4 mice infected with T. b. rhodesiense STIB900 with four daily 5 mg/kg intraperitoneal doses, as well as with single doses of ≤ 10 mg/kg. Derivatives 5 and 28 (prodrugs of 1 and 25) each cured 3/4 mice with four daily 25 mg/kg oral doses.

Animal models of efficacy to accelerate drug discovery in malaria

The emergence of resistance to artemisinins and the renewed efforts to eradicate malaria demand the urgent development of new drugs. In this endeavour, the evaluation of efficacy in animal models is often a go/no go decision assay in drug discovery. This important role relies on the capability of animal models to assess the disposition, toxicology and efficacy of drugs in a single test. Although the relative merits of each efficacy model of malaria as human surrogate have been extensively discussed, there are no critical analyses on the use of such models in current drug discovery. In this article, we intend to analyse how efficacy models are used to discover new antimalarial drugs. Our analysis indicates that testing drug efficacy is often the last assay in each discovery stage and the experimental designs utilized are not optimized to expedite decision-making and inform clinical development. In light of this analysis, we propose new ways to accelerate drug discovery using efficacy models.

Designing the next generation of medicines for malaria control and eradication

In the fight against malaria new medicines are an essential weapon. For the parts of the world where the current gold standard artemisinin combination therapies are active, significant improvements can still be made: for example combination medicines which allow for single dose regimens, cheaper, safer and more effective medicines, or improved stability under field conditions. For those parts of the world where the existing combinations show less than optimal activity, the priority is to have activity against emerging resistant strains, and other criteria take a secondary role. For new medicines to be optimal in malaria control they must also be able to reduce transmission and prevent relapse of dormant forms: additional constraints on a combination medicine. In the absence of a highly effective vaccine, new medicines are also needed to protect patient populations. In this paper, an outline definition of the ideal and minimally acceptable characteristics of the types of clinical candidate molecule which are needed (target candidate profiles) is suggested. In addition, the optimal and minimally acceptable characteristics of combination medicines are outlined (target product profiles). MMV presents now a suggested framework for combining the new candidates to produce the new medicines. Sustained investment over the next decade in discovery and development of new molecules is essential to enable the long-term delivery of the medicines needed to combat malaria.

Quinolone-3-diarylethers: a new class of antimalarial drug

The goal for developing new antimalarial drugs is to find a molecule that can target multiple stages of the parasite's life cycle, thus impacting prevention, treatment, and transmission of the disease. The 4(1H)-quinolone-3-diarylethers are selective potent inhibitors of the parasite's mitochondrial cytochrome bc1 complex. These compounds are highly active against the human malaria parasites Plasmodium falciparum and Plasmodium vivax. They target both the liver and blood stages of the parasite as well as the forms that are crucial for disease transmission, that is, the gametocytes, the zygote, the ookinete, and the oocyst. Selected as a preclinical candidate, ELQ-300 has good oral bioavailability at efficacious doses in mice, is metabolically stable, and is highly active in blocking transmission in rodent models of malaria. Given its predicted low dose in patients and its predicted long half-life, ELQ-300 has potential as a new drug for the treatment, prevention, and, ultimately, eradication of human malaria.

Evaluation of bioluminescence-based assays of anti-malarial drug activity

Background

Transgenic Plasmodium falciparum expressing luciferase offers an attractive bioluminescence-based assay platform for the investigation of the pharmacological properties of anti-malarial drugs. Here a side-by-side comparison of bioluminescence and fluorescence-based assays, utilizing a luciferase reporter cassette that confers a strong temporal pattern of luciferase expression during the S-phase of intraerythrocytic development, is reported.

Methods

Key assay parameters for a range of commercially available luminogenic substrates are determined and compared to those measured using a Malaria Sybr Green I fluorescence assay. In addition, the short-term temporal effects of anti-malarial compounds are evaluated using both bioluminescent and fluorescent assay platforms.

Results

The Z’, % coefficient of variation and 50% inhibition concentrations are essentially the same for bioluminescent and fluorescent assays in transgenic parasites generated in both chloroquine-sensitive and -resistant genetic backgrounds. Bioluminescent assays, irrespective of the luminogenic agent employed, do, however, offer significantly enhanced signal-to-noise ratios. Moreover, the bioluminescent assay is more dynamic in terms of determining temporal effects immediately following drug perturbation.

Conclusion

This study suggests that opportunities for bioluminescence-based assays lie not in the measurement of 50% inhibition concentrations, where the cheaper fluorescence assay performs excellently and is not restricted by the need to genetically modify the parasite clone under investigation. Instead, assays that use the dynamic response of the luciferase reporter for semi-automated screening of additional pharmacological properties, such as relative rate-of-kill and lethal dose estimation, are a more attractive development opportunity.

Development of humanized mouse models to study human malaria parasite infection

Malaria is a disease caused by infection with Plasmodium parasites that are transmitted by mosquito bite. Five different species of Plasmodium infect humans with severe disease, but human malaria is primarily caused by Plasmodium falciparum. The burden of malaria on the developing world is enormous, and a fully protective vaccine is still elusive. One of the biggest challenges in the quest for the development of new antimalarial drugs and vaccines is the lack of accessible animal models to study P. falciparum infection because the parasite is restricted to the great apes and human hosts. Here, we review the current state of research in this field and provide an outlook of the development of humanized small animal models to study P. falciparum infection that will accelerate fundamental research into human parasite biology and could accelerate drug and vaccine design in the future.

Know your enemy: understanding the role of PfCRT in drug resistance could lead to new antimalarial tactics

The prevention and treatment of malaria is heavily dependent on antimalarial drugs. However, beginning with the emergence of chloroquine (CQ)-resistant Plasmodium falciparum parasites 50 years ago, efforts to control the disease have been thwarted by failed or failing drugs. Mutations in the parasite's 'chloroquine resistance transporter' (PfCRT) are the primary cause of CQ resistance. Furthermore, changes in PfCRT (and in several other transport proteins) are associated with decreases or increases in the parasite's susceptibility to a number of other antimalarial drugs. Here, we review recent advances in our understanding of CQ resistance and discuss these in the broader context of the parasite's susceptibilities to other quinolines and related drugs. We suggest that PfCRT can be viewed both as a 'multidrug-resistance carrier' and as a drug target, and that the quinoline-resistance mechanism is a potential 'Achilles' heel' of the parasite. We examine a number of the antimalarial strategies currently undergoing development that are designed to exploit the resistance mechanism, including relatively simple measures, such as alternative CQ dosages, as well as new drugs that either circumvent the resistance mechanism or target it directly.

Antimalarial activity of 9a-N substituted 15-membered azalides with improved in vitro and in vivo activity over azithromycin

Novel classes of antimalarial drugs are needed due to emerging drug resistance. Azithromycin, the first macrolide investigated for malaria treatment and prophylaxis, failed as a single agent and thus novel analogues were envisaged as the next generation with improved activity. We synthesized 42 new 9a-N substituted 15-membered azalides with amide and amine functionalities via simple and inexpensive chemical procedures using easily available building blocks. These compounds exhibited marked advances over azithromycin in vitro in terms of potency against Plasmodium falciparum (over 100-fold) and high selectivity for the parasite and were characterized by moderate oral bioavailability in vivo. Two amines and one amide derivative showed improved in vivo potency in comparison to azithromycin when tested in a mouse efficacy model. Results obtained for compound 6u, including improved in vitro potency, good pharmacokinetic parameters, and in vivo efficacy higher than azithromycin and comparable to chloroquine, warrant its further development for malaria treatment and prophylaxis.

In vitro and in vivo antimalarial activities of T-2307, a novel arylamidine

T-2307, a novel arylamidine, has been shown to exhibit broad-spectrum antifungal activities against clinically significant pathogens. Here, we evaluated the in vitro and in vivo antimalarial activity of T-2307. The 50% inhibitory concentrations (IC₅₀s) of T-2307 against Plasmodium falciparum FCR-3 and K-1 strains were 0.47 and 0.17 μM, respectively. T-2307 at 2.5 to 10 mg/kg of body weight/day exhibited activity against blood stage and liver stage parasites in rodent malaria models. In conclusion, T-2307 exhibited in vitro and in vivo antimalarial activity.

Challenges in antimalarial drug discovery

Malaria is one of the most devastating diseases in the world, affecting almost 225 million people a year, and causing over 780,000 deaths, most of which are children under the age of 5 years. Following the recent call for the eradication of the disease, supported by the WHO, there has been increasing investment into antimalarial drug-discovery projects. These activities are aimed at generating the next generation of molecules focused on the treatment and transmission-blocking of Plasmodium falciparum and Plasmodium vivax endo- and exo-erythrocytic stages of the parasite. This article summarizes the current top-level thinking regarding the prosecution of such endeavors and the disease-specific considerations in project planning.

Cyclopropyl Carboxamides: A New Oral Antimalarial Series Derived from the Tres Cantos Anti-Malarial Set (TCAMS)

Rapid triaging of three series of related hits selected from the Tres Cantos Anti-Malarial Set (TCAMS) are described. A triazolopyrimidine series was deprioritized due to delayed inhibition of parasite growth. A lactic acid series has derivatives with IC50 < 500 nM in a standard Plasmodium falciparum in vitro whole cell assay (Pf assay) but shows half-lives of < 30 min in both human and murine microsomes. Compound 19, from a series of cyclopropyl carboxamides, is a highly potent in vitro inhibitor of P. falciparum (IC50 = 3 nM) and has an oral bioavailability of 55% in CD-1 mice and an ED90 of 20 mg/kg after oral dosing in a nonmyelo-depleted P. falciparum murine model.

Cyclopropyl carboxamides, a chemically novel class of antimalarial agents identified in a phenotypic screen

Malaria is one of the deadliest infectious diseases in the world, with the eukaryotic parasite Plasmodium falciparum causing the most severe form of the disease. Discovery of new classes of antimalarial drugs has become an urgent task to counteract the increasing problem of drug resistance. Screening directly for compounds able to inhibit parasite growth in vitro is one of the main approaches the malaria research community is now pursuing for the identification of novel antimalarial drug leads. Very recently, thousands of compounds with potent activity against the parasite P. falciparum have been identified and information about their molecular descriptors, antiplasmodial potency, and cytotoxicity is publicly available. Now the challenges are how to identify the most promising chemotypes for further development and how best to progress these compounds through a lead optimization program to generate antimalarial drug candidates. We report here the first chemical series to be characterized from one of those screenings, a completely novel chemical class with the generic name cyclopropyl carboxamides that has never before been described as having antimalarial or other pharmacological activities. Cyclopropyl carboxamides are potent inhibitors of drug-sensitive and -resistant strains of P. falciparum in vitro and show in vivo oral efficacy in malaria mouse models. In the present work, we describe the biological characterization of this chemical family, showing that inhibition of their still unknown target has very favorable pharmacological consequences but the compounds themselves seem to select for resistance at a high frequency.

Structure-guided lead optimization of triazolopyrimidine-ring substituents identifies potent Plasmodium falciparum dihydroorotate dehydrogenase inhibitors with clinical candidate potential

Drug therapy is the mainstay of antimalarial therapy, yet current drugs are threatened by the development of resistance. In an effort to identify new potential antimalarials, we have undertaken a lead optimization program around our previously identified triazolopyrimidine-based series of Plasmodium falciparum dihydroorotate dehydrogenase (PfDHODH) inhibitors. The X-ray structure of PfDHODH was used to inform the medicinal chemistry program allowing the identification of a potent and selective inhibitor (DSM265) that acts through DHODH inhibition to kill both sensitive and drug resistant strains of the parasite. This compound has similar potency to chloroquine in the humanized SCID mouse P. falciparum model, can be synthesized by a simple route, and rodent pharmacokinetic studies demonstrated it has excellent oral bioavailability, a long half-life and low clearance. These studies have identified the first candidate in the triazolopyrimidine series to meet previously established progression criteria for efficacy and ADME properties, justifying further development of this compound toward clinical candidate status.

Aminoindoles, a novel scaffold with potent activity against Plasmodium falciparum

This study characterizes aminoindole molecules that are analogs of Genz-644442. Genz-644442 was identified as a hit in a screen of ~70,000 compounds in the Broad Institute's small-molecule library and the ICCB-L compound collection at Harvard Medical School. Genz-644442 is a potent inhibitor of Plasmodium falciparum in vitro (50% inhibitory concentrations [IC₅₀s], 200 to 285 nM) and inhibits P. berghei in vivo with an efficacy of > 99% in an adapted version of Peters' 4-day suppressive test (W. Peters, Ann. Trop. Med. Parasitol. 69:155-171, 1975). Genz-644442 became the focus of medicinal chemistry optimization; 321 analogs were synthesized and were tested for in vitro potency against P. falciparum and for in vitro absorption, distribution, metabolism, and excretion (ADME) properties. This yielded compounds with IC₅₀s of approximately 30 nM. The lead compound, Genz-668764, has been characterized in more detail. It is a single enantiomer with IC₅₀s of 28 to 65 nM against P. falciparum in vitro. In the 4-day P. berghei model, when it was dosed at 100 mg/kg of body weight/day, no parasites were detected on day 4 postinfection. However, parasites recrudesced by day 9. Dosing at 200 mg/kg/day twice a day resulted in cures of 3/5 animals. The compound had comparable activity against P. falciparum blood stages in a human-engrafted NOD-scid mouse model. Genz-668764 had a terminal half-life of 2.8 h and plasma trough levels of 41 ng/ml when it was dosed twice a day orally at 55 mg/kg/day. Seven-day rat safety studies showed a no-observable-adverse-effect level (NOAEL) at 200 mg/kg/day; the compound was not mutagenic in Ames tests, did not inhibit the hERG channel, and did not have potent activity against a broad panel of receptors and enzymes. Employing allometric scaling and using in vitro ADME data, the predicted human minimum efficacious dose of Genz-668764 in a 3-day once-daily dosing regimen was 421 mg/day/70 kg, which would maintain plasma trough levels above the IC₉₀ against P. falciparum for at least 96 h after the last dose. The predicted human therapeutic index was approximately 3, on the basis of the exposure in rats at the NOAEL. We were unable to select for parasites with >2-fold decreased sensitivity to the parent compound, Genz-644442, over 270 days of in vitro culture under drug pressure. These characteristics make Genz-668764 a good candidate for preclinical development.

Methodology and application of flow cytometry for investigation of human malaria parasites

Historically, examinations of the inhibition of malaria parasite growth/invasion, whether using drugs or antibodies, have relied on the use of microscopy or radioactive hypoxanthine uptake. These are considered gold standards for measuring the effectiveness of antimalarial treatments, however, these methods have well known shortcomings. With the advent of flow cytometry coupled with the use of fluorescent DNA stains allowed for increased speed, reproducibility, and qualitative estimates of the effectiveness of antibodies and drugs to limit malaria parasite growth which addresses the challenges of traditional techniques. Because materials and machines available to research facilities are so varied, different methods have been developed to investigate malaria parasites by flow cytometry. This review is intended to serve as a reference guide for advanced users and importantly, as a primer for new users, to support expanded use and improvements to malaria flow cytometry, particularly in endemic countries.

Further improvements of the P. falciparum humanized mouse model

BACKGROUND

It has been shown previously that it is possible to obtain growth of Plasmodium falciparum in human erythrocytes grafted in mice lacking adaptive immune responses by controlling, to a certain extent, innate defences with liposomes containing clodronate (clo-lip). However, the reproducibility of those models is limited, with only a proportion of animals supporting longstanding parasitemia, due to strong inflammation induced by P. falciparum. Optimisation of the model is much needed for the study of new anti-malarial drugs, drug combinations, and candidate vaccines.

MATERIALS/METHODS

We investigated the possibility of improving previous models by employing the intravenous route (IV) for delivery of both human erythrocytes (huRBC) and P. falciparum, instead of the intraperitoneal route (IP), by testing various immunosuppressive drugs that might help to control innate mouse defences, and by exploring the potential benefits of using immunodeficient mice with additional genetic defects, such as those with IL-2Rγ deficiency (NSG mice).

RESULTS

We demonstrate here the role of aging, of inosine and of the IL-2 receptor γ mutation in controlling P. falciparum induced inflammation. IV delivery of huRBC and P. falciparum in clo-lip treated NSG mice led to successful infection in 100% of inoculated mice, rapid rise of parasitemia to high levels (up to 40%), long-lasting parasitemia, and consistent results from mouse-to-mouse. Characteristics were closer to human infection than in previous models, with evidence of synchronisation, partial sequestration, and receptivity to various P. falciparum strains without preliminary adaptation. However, results show that a major IL-12p70 inflammatory response remains prevalent.

CONCLUSION

The combination of the NSG mouse, clodronate loaded liposomes, and IV delivery of huRBC has produced a reliable and more relevant model that better meets the needs of Malaria research.

The relevance of non-human primate and rodent malaria models for humans

At the 2010 Keystone Symposium on "Malaria: new approaches to understanding Host-Parasite interactions", an extra scientific session to discuss animal models in malaria research was convened at the request of participants. This was prompted by the concern of investigators that skepticism in the malaria community about the use and relevance of animal models, particularly rodent models of severe malaria, has impacted on funding decisions and publication of research using animal models. Several speakers took the opportunity to demonstrate the similarities between findings in rodent models and human severe disease, as well as points of difference. The variety of malaria presentations in the different experimental models parallels the wide diversity of human malaria disease and, therefore, might be viewed as a strength. Many of the key features of human malaria can be replicated in a variety of nonhuman primate models, which are very under-utilized. The importance of animal models in the discovery of new anti-malarial drugs was emphasized. The major conclusions of the session were that experimental and human studies should be more closely linked so that they inform each other, and that there should be wider access to relevant clinical material.

Feasibility of flow cytometry for measurements of Plasmodium falciparum parasite burden in studies in areas of malaria endemicity by use of bidimensional assessment of YOYO-1 and autofluorescence

The detection and quantification of Plasmodium falciparum in studies of malaria endemicity primarily relies upon microscopy. High-throughput quantitative methods with less subjectivity and greater reliability are needed for investigational studies. The staining of parasitized erythrocytes with YOYO-1 for flow cytometry bears great potential as a tool for assessing malaria parasite burden. Capillary blood was collected from children presenting to the pediatric ward of the Manhiça District Hospital in Mozambique for parasitemia assessment by thick and thin blood films, flow cytometry (YOYO-1(530/585)), and quantitative real-time PCR (qRT-PCR). Whole blood was fixed and stained with YOYO-1 for acquisition on a cytometer to assess the frequency of infected erythrocyte events. qRT-PCR was used as the gold standard for the detection of P. falciparum. The YOYO-1(530/585) method was as sensitive and specific as conventional microscopy (area under the receiver operating characteristic, 0.9 for both methods). The interrater mean difference for YOYO-1(530/585) was near zero. Parasite density using flow cytometry and complete blood counts returned density estimates with a mean difference 2.2 times greater than results by microscopy (confidence interval, 1.46 to 3.60) but with limits of agreement between 10 times lower and 50 times higher than those of microscopy. The YOYO-1(530/585) staining pattern was established exactly as demonstrated in animal models, but the assay was limited by the lack of appropriate negative-control samples for establishing background levels and the definition of positives in areas in which malaria is endemic. YOYO-1(530/585) is a high-throughput tool with great potential if the limitations of negative controls and heterogeneous levels of background signal can be overcome.

Falcipains and other cysteine proteases of malaria parasites

A number of cysteine proteases of malaria parasites have been described and many more are suggested by analysis of the Plasmodium falciparum genome sequence. The best characterized of these proteases are the falcipains, a family of four papain-family enzymes. Falcipain-2 and falcipain-3 act in concert with other proteases to hydrolyze host erythrocyte hemoglobin in the parasite food vacuole. Disruption of the falcipain-2 gene led to a transient block in hemoglobin hydrolysis and parasites with increased sensitivity to protease inhibitors. Disruption of the falcipain-3 gene was not possible, strongly suggesting that this protease is essential for erythrocytic parasites. Disruption of the falcipain-1 gene did not alter development in erythrocytes, but led to decreased production of oocysts in mosquitoes. other papain-family proteases predicted by the genome sequence include dipeptidyl peptidases, a calpain homolog and serine-repeat antigens (SERAs). Dipeptidyl aminopeptidase 1 appears to be essential and localized to the food vacuole, suggesting a role in hemoglobin hydrolysis. Dipeptidyl aminopeptidase 3 appears to play a role in the rupture of erythrocytes by mature parasites. the P. falciparum calpain homolog gene could not be disrupted, suggesting that the protein is essential and a role in the parasite cell cycle has been suggested. Nine P. falciparum SERAs have cysteine protease motifs, but in some the active site cys is replaced by a Ser. Gene disruption studies suggested that SERA-5 and SERA-6 are essential. activation of SERA-5 by a serine protease seems to be required for merozoite egress from the erythrocyte. New drugs for malaria are greatly needed and cysteine proteases represent potential drug targets. cysteine protease inhibitors have demonstrated potent antimalarial effects and the optimization and testing of falcipain inhibitor antimalarials is underway.

Novel inhibitors of Plasmodium falciparum dihydroorotate dehydrogenase with anti-malarial activity in the mouse model

Plasmodium falciparum, the causative agent of the most deadly form of human malaria, is unable to salvage pyrimidines and must rely on de novo biosynthesis for survival. Dihydroorotate dehydrogenase (DHODH) catalyzes the rate-limiting step in the pyrimidine biosynthetic pathway and represents a potential target for anti-malarial therapy. A high throughput screen and subsequent medicinal chemistry program identified a series of N-alkyl-5-(1H-benzimidazol-1-yl)thiophene-2-carboxamides with low nanomolar in vitro potency against DHODH from P. falciparum, P. vivax, and P. berghei. The compounds were selective for the parasite enzymes over human DHODH, and x-ray structural data on the analog Genz-667348, demonstrated that species selectivity could be attributed to amino acid differences in the inhibitor-binding site. Compounds from this series demonstrated in vitro potency against the 3D7 and Dd2 strains of P. falciparum, good tolerability and oral exposure in the mouse, and ED(50) values in the 4-day murine P. berghei efficacy model of 13-21 mg/kg/day with oral twice-daily dosing. In particular, treatment with Genz-667348 at 100 mg/kg/day resulted in sterile cure. Two recent analogs of Genz-667348 are currently undergoing pilot toxicity testing to determine suitability as clinical development candidates.

Analysis of innate defences against Plasmodium falciparum in immunodeficient mice

BACKGROUND

Mice with genetic deficiencies in adaptive immunity are used for the grafting of human cells or pathogens, to study human diseases, however, the innate immune responses to xenografts in these mice has received little attention. Using the NOD/SCID Plasmodium falciparum mouse model an analysis of innate defences responsible for the substantial control of P. falciparum which remains in such mice, was performed.

METHODS

NOD/SCID mice undergoing an immunomodulatory protocol that includes, clodronate-loaded liposomes to deplete macrophages and an anti-polymorphonuclear leukocytes antibody, were grafted with human red blood cells and P. falciparum. The systematic and kinetic analysis of the remaining innate immune responses included the number and phenotype of peripheral blood leukocytes as well as inflammatory cytokines/chemokines released in periphery. The innate responses towards the murine parasite Plasmodium yoelii were used as a control.

RESULTS

Results show that 1) P. falciparum induces a strong inflammation characterized by an increase in circulating leukocytes and the release of inflammatory cytokines; 2) in contrast, the rodent parasite P. yoelii, induces a far more moderate inflammation; 3) human red blood cells and the anti-inflammatory agents employed induce low-grade inflammation; and 4) macrophages seem to bear the most critical function in controlling P. falciparum survival in those mice, whereas polymorphonuclear and NK cells have only a minor role.

CONCLUSIONS

Despite the use of an immunomodulatory treatment, immunodeficient NOD/SCID mice are still able to mount substantial innate responses that seem to be correlated with parasite clearance. Those results bring new insights on the ability of innate immunity from immunodeficient mice to control xenografts of cells of human origin and human pathogens.

Improved murine model of malaria using Plasmodium falciparum competent strains and non-myelodepleted NOD-scid IL2Rgammanull mice engrafted with human erythrocytes

Murine models of Plasmodium falciparum malaria may become crucial tools in drug discovery. Here we show that non-myelodepleted NOD-scid IL2Rgamma(null) mice engrafted with human erythrocytes support an infectious burden up to tenfold higher than that supported by engrafted NOD-scid beta2microglobulin(null) mice. The new model was validated for drug discovery and was used to assess the therapeutic efficacy of 4-pyridones, selective inhibitors of P. falciparum cytochrome bc1.

Quantitative measurement of Plasmodium-infected erythrocytes in murine models of malaria by flow cytometry using bidimensional assessment of SYTO-16 fluorescence

Flow cytometry is a powerful tool for measuring parasitemias in murine malaria models used to test new antimalarials. Measurement of the emission of the nonpermeable nucleic acid dye YOYO-1 (at 530 and 585 nm after excitation at 488 nm) allowed the unambiguous detection of low parasitemias (> or =0.01%) but required prolonged fixation and permeabilization of the sample. Thus, we tested whether this issue could be overcome by use of the cell-permeant dye SYTO-16 with this same bidimensional method. Blood samples from CD1 mice infected with Plasmodium yoelii, Plasmodium vinckei, or Plasmodium chabaudi or from NOD(scidbeta2m-/-) engrafted with human erythrocytes and infected with P. falciparum were stained with SYTO-16 in the presence or absence of TER-119 mAb (for engrafted mice) in 96-well plate format and acquired in Trucount tubes. Bidimensional analysis with SYTO-16 was quantitatively equivalent to YOYO-1. Moreover, by combining SYTO-16 with the use of TER-119-PE antimouse erythrocyte mAb and Trucount tubes, the measurement of the concentration of P. falciparum-infected erythrocytes over a range of five orders of magnitude was achieved. Bidimensional analysis using SYTO-16 can be used to accurately measure the concentration of Plasmodium spp.-infected erythrocytes in mice without complex sample preparation.

Recent highlights in antimalarial drug resistance and chemotherapy research

This review summarizes recent investigations into antimalarial drug resistance and chemotherapy, including reports of some of the many exciting talks and posters on this topic that were presented at the third Molecular Approaches to Malaria meeting held in Lorne, Australia, in February 2008 (MAM 2008). After surveying this area of research, we focus on two important questions: what is the molecular contribution of pfcrt to chloroquine resistance, and what is the mechanism of action of artemisinin? We conclude with thoughts about the current state of antimalarial chemotherapy and priorities moving forward.